1. Field of the Invention
The present invention relates generally to the preparation of polyurethane and polyisocyanurate polymers, and more particularly to the preparation of low viscosity aromatic polyols which can be reacted with polyisocyanates to form such polymers.
2. Description of the Prior Art
Polyurethane foams are formed by reaction between an isocyanate and a polyol having at least two hydroxyl groups. For polyisocyanurate foams, the molar ratio of isocyanate to polyol will be greater than 1 to 1, usually equal to or greater than 2 to 1. Polyurethane and polyisocyanurate foams are produced by the reaction of the isocyanate and the polyol with a suitable blowing agent, surfactant and catalyst. The degree of rigid compressive strength or flexible load deflection is determined by the functionality and molecular weight of the isocyanate and the polyol.
Aromatic polyester polyols useful for the production of polyurethane and polyisocyanurate foams can be derived from polyalkylene terephthalate polymers, such as polyethylene terephthalate (PET), by digestion of the polymer with various low molecular weight aliphatic polyols, such as polyhydric alcohols, ethers, and mixed ether-ester polyols.
The use of aromatic ester polyols obtained from polyalkylene terephthalate polymers for the preparation of polyurethanes and polyisocyanurates has a number of advantages. First, the polyalkylene terephthalate polymers can be obtained from scrap materials such as used photographic film, synthetic fibers, plastic bottles such as those used in the soft drink industry, and waste from the production of other products made from polyalkylene terephthalate polymers. A second advantage is found in the increased resiliency of flexible polyurethane foams and increased compressive strength of rigid foams prepared from such digested polyols in comparison to those prepared from other polyols. Finally, it has been found that polyurethanes and polyisocyanurates formed from such digested polyols have improved fire retardancy when compared to other conventional foams.
Despite these advantages, polyols produced by digesting polyalkylene terephthalate polymers suffer from a number of disadvantages. The digestion products are highly viscous, typically having viscosities in the range from about 20,000 cps to solid at room temperature. In order to lower the viscosity, the digested polyols are usually blended with excess amounts of a low molecular weight aliphatic polyol or reacted with isocyanates to form prepolymers. Although this lowers the overall viscosity of the combined polyols or prepolymers, the resulting mixtures are unstable and often crystallize after short term storage. In addition, the digested polyols have limited solubility in fluorocarbon blowing agents and are dimensionally unstable.
U.S. Pat. No. 4,048,104 to Svoboda et al. describes a method for utilizing polyalkylene terephthalate digestion product in the preparation of flexible polyurethane foams. Polyisocyanate prepolymers are prepared by reacting an organic polyisocyanate, e.g., toluene diisocyanate, with the digested polyol to form a prepolymer which is substantially free of unreacted hydroxyl radicals. The polyisocyanate prepolymer is then combined with additional polyol, a suitable blowing agent, a surfactant, and a catalyst in order to produce the desired flexible polyurethane.
U.S. Pat. No. 4,223,068 to Carlstrom et al. discloses the use of a similar polyol digestion product in the production of rigid polyurethane foams. In order to provide a suitable organic polyol, the polyol digestion product of a polyalkylene terephthalate is combined with a low molecular weight polyol. The amount of the digestion product present relative to the total organic polyol can be no more than 30% in order to retain the desired physical properties of the foam, particularly compressive strength and dimensional stability. While the patent to Carlstrom et al. discloses a suitable organic polyol, it suffers from the disadvantages associated with the presence of high concentrations of a low molecular weight polyol described above.
It would thus be desirable to provide an aromatic polyol derived from the digestion of polyalkylene terephthlates which is suitable for use as the sole polyol in producing polyurethane and polyisocyanurate foams.